The ALMaQUEST Survey XV: The Dependence of the Molecular-to-Atomic Gas Ratios on Resolved Optical Diagnostics

The atomic-to-molecular gas conversion is a critical step in the baryon cycle of galaxies, which sets the initial conditions for subsequent star formation and influences the multi-phase interstellar medium. We compiled a sample of 94 nearby galaxies with observations of multi-phase gas contents by utilizing public H I, CO, and optical IFU data from the MaNGA survey together with new FAST H I observations. In agreement with previous results, our sample shows that the global molecular-to-atomic gas ratio (R_ mol≡ log M_ H_2/M_ H I) is correlated with the global stellar mass surface density μ_* with a Kendall's τ coefficient of 0.25 and p < 10^-3, less tightly but still correlated with stellar mass and NUV- r color, and not related to the specific star formation rate (sSFR). The cold gas distribution and kinematics inferred from the H I and CO global profile asymmetry and shape do not significantly rely on R_ mol. Thanks to the availability of kpc-scale observations of MaNGA, we decompose galaxies into H II, composite, and AGN-dominated regions by using the BPT diagrams. With increasing R_ mol, the fraction of H II regions within 1.5 effective radius decreases slightly; the density distribution in the spatially resolved BPT diagram also changes significantly, suggesting changes in metallicity and ionization states. Galaxies with high R_ mol tend to have high oxygen abundance, both at one effective radius with a Kendall's τ coefficient of 0.37 (p < 10^-3) and their central regions. Among all parameters investigated here, the oxygen abundance at one effective radius has the strongest relation with global R_ mol, but the dependence of gas conversion on gas distribution and galaxy ionization states is weak.